This invention relates to a multistage tenter used to process belt-shaped sheets or webs such as belt-shaped films.
A conventional multistage tenter of this type is known as shown in FIG. 10. The multistage tenter has a drying unit 1 in which endless clip chains 2 are laid over sprockets 3, 4, 5, 6, and 7. The sprockets 6 are driven by a drive source (not shown), so that two clip chains 2 and 2 engaged with these sprockets 3 through 7 as shown in FIG. 11 are driven, whereby a belt-shaped film 8 (hereinafter referred to merely as "a film 8", when applicable) is inserted into the drying unit 1 through a film inlet formed in its side wall. The film 8 thus inserted is conveyed downwardly while changing the direction of movement (or being bent) so that it is dried in the drying unit. The film thus dried is moved out of the drying unit passing through a film removing roll 9.
The clip chains 2 are constructed as shown in FIGS. 12 and 13. Two coupling pins 11 and 11 are secured to the side walls 10a of each clip body 10. Rotors 12 and 12 are provided at first ends of the coupling pins 11 in such a manner that they are disposed between the confronted surfaces of guide rails 13 and 13 which are provided above and below the rotors 12 and 12. A pin plate 14 is coupled to the other ends of the coupling pins 11. Rotors 15 and 15 and rotors 16 and 16 are provided obliquely above and obliquely below each of the rotors 12 in such a manner that they are abutted against the guide rails 13, as shown in FIGS. 12 and 13.
Each coupling pin 11 is inserted into a bushing 17, which is inserted into a roller 18. A pair of chain links 19 and 19 are mounted on the bushing 17 in such a manner that they are located on both sides of the roller 18. In FIG. 12, reference character 3a designates the teeth of the sprockets. The sprockets 4, 5 and 6 adapted to change the direction of movement of the clip chains 2 are equal in construction to one another. As shown in FIGS. 14 and 15, the right and left sprockets 4 are supported through bearings 21 on shafts 20, and distance adjusting device 22 for adjusting the distance between the right and left sprockets 4 is provided below the bearings 22, respectively.
Each of the right and left sprockets 7 is provided with a tensioning mechanism 23 which is as shown in FIGS. 16 and 17. This will be described in more detail. The sprocket 7 and a hydraulic cylinder 24 are arranged in the middle of a rectangular frame 25 in such a manner that they are spaced from each other and are in parallel with each other. Four sprocket holders 26 are arranged near the four corners of the frame 25, respectively. Chains 28 and 28 are connected under tension, with the aid of the sprocket holders 26, between the piston rods 24a and 24a protruded from both ends of the hydraulic cylinder 24 and both sides of a bearing frame 27 provided below the sprocket 7, respectively. The tension of the chains 28 is detected by a tension detecting load cell 29.
In the conventional multistage tenter thus constructed, each of the clip chains is of the order of 40 to 60 meters. With the conventional multistage tenter, in order to reduce the time required for drying a film, it is necessary to increase the heating temperature. However, increase of the heating temperature may wrinkle the surface of the film. In order to effectively dry the film at a suitable, relatively low temperature, it is necessary to increase the length of the heating section, and accordingly to increase the length of the clip chains. If each clip chain is increased in length, then the number of chain links forming the clip chain is increased accordingly, with the result that the pitch errors of the chain links are accumulated, providing a large accumulative pitch error as a whole. This large accumulative pitch error will result in the difference in length between the right and left clip chains. The difference should be 10 mm or less per 100 m with the clip chains laid between the film inlet and the film outlet. However, in the case where the clip chain is of the order of 200 m, it is rather difficult to reduce the difference to 10 mm or less. In order to meet the requirement, a method is employed in which chain links substantially equal in hole pitch are selected as pairs, which are distributed uniformly to the right and left clip chains. However, to do so would take a lot of time and labor. On the other hand, in the multistage tenter, the effect of the difference between the right and left clip chains is large where the direction of movement of the clip chains is changed by the sprockets.
Furthermore in the conventional multistage tenter, the right and left sprockets for changing the direction of movement of the clip chains are provided individually; that is, they are rotated separately, and accordingly the movements of the right and left clip chains are not synchronous with each other. Hence, the accumulative pitch errors of the right and left clip chains are not absorbed, and the difference in length of the right and left clip chains, which is due to the accumulative pitch errors, will form wrinkles on the film.
The clip chain tensioning mechanisms are provided for the right and left sprockets, respectively. The tensions of the right and left clip chains are detected, so that the clip chain shorter is tensioned by the respective hydraulic cylinder more than the other. However, it is rather difficult to adjust the tensions of the clip chains with their own hydraulic cylinders separately thereby to absorb the difference in length therebetween in order to prevent the formation of wrinkles on the film.